1. Field of the Invention
The present invention relates generally to an apparatus and method for analog to digital conversion, and in a particular to analog to digital voltage conversion using a ramp converter.
2. Discussion of the Related Art
Analog to digital converters (ADCs) are used in image sensors to convert an analog voltage signal representing the light intensity sensed by a photodiode into a digital signal to be stored and processed by digital processing circuits. The resolution of the resulting digital signal is determined by the number of bits generated by the ADC.
Ramp converters are a type of ADC that use a voltage ramp to detect the level of the analog input signal. While the level of the voltage ramp increases, a counter is incremented, and when the level of the voltage ramp matches the analog input signal, the digital value of the counter at that instant provides the digital conversion of the input signal.
There is today a demand for image sensors having a high data resolution, for example having pixel values of at least 10 bits, combined with a high image resolution, for example between 5 and 20 Megapixels, and a high frame rate. However, as the number of bits increases, the size and complexity of the ADC also increases. Furthermore, the greater the number of bits of the converter, the slower its operation. Indeed, the number of bits determines the maximum count value of the counter. Thus, assuming a given clock frequency of the counter, the higher the number of bits, the longer the counter will take to reach its maximum count value. Indeed, for relatively large matrices, the frame rate at full resolution is limited.
Current ADCs do not adequately address the above demands, and/or result in the introduction of undesirable distortion in the digital signal. There is thus a need for an improved ADC that addresses these issues.